Remove now-unnecessary wrappers around gfx::Screen::GetScreen()

ash/display/screen_ash.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/display/screen_ash.h"
 
 #include "ash/display/display_manager.h"
 #include "ash/display/window_tree_host_manager.h"
 #include "ash/root_window_controller.h"
 #include "ash/root_window_settings.h"
@@ skipping 81 matching lines @@
 void ScreenAsh::NotifyDisplayRemoved(const gfx::Display& display) {
   FOR_EACH_OBSERVER(
       gfx::DisplayObserver, observers_, OnDisplayRemoved(display));
 }
 
 gfx::Point ScreenAsh::GetCursorScreenPoint() {
   return aura::Env::GetInstance()->last_mouse_location();
 }
 
 gfx::NativeWindow ScreenAsh::GetWindowUnderCursor() {
-  return GetWindowAtScreenPoint(Shell::GetScreen()->GetCursorScreenPoint());
+  return GetWindowAtScreenPoint(
+      gfx::Screen::GetScreen()->GetCursorScreenPoint());
 }
 
 gfx::NativeWindow ScreenAsh::GetWindowAtScreenPoint(const gfx::Point& point) {
   aura::Window* root_window = wm::GetRootWindowAt(point);
   aura::client::ScreenPositionClient* position_client =
       aura::client::GetScreenPositionClient(root_window);
 
   gfx::Point local_point = point;
   if (position_client)
     position_client->ConvertPointFromScreen(root_window, &local_point);
@@ skipping 63 matching lines @@
 
 void ScreenAsh::RemoveObserver(gfx::DisplayObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
 gfx::Screen* ScreenAsh::CloneForShutdown() {
   return new ScreenForShutdown(this);
 }
 
 }  // namespace ash